Changing a SIM card in an Iphone 3GS?

I'm currently using a regular phone that I got with a plan from AT&T and I'm trying to change over to an Iphone 3GS. I know I can just change the SIM cards out, but I don't want AT&T to know that I have a smart phone so they won;t charge me the extra fee. Is there a way to do that? Jailbreak?

Not sure what youre asking

This requires changing the IMEI of the phone (hardware serial number). Doing this for the reason that you have specified is illegal as you would be stealing service. If you don't want to pay extra for having a smart phone you should unlock your phone and find a carrier that doesn't charge extra for it.